In practical cryptography, a Physical Unclonable Function or PUF is a function that is embodied in a physical structure of a die or a chip and may be easy to evaluate but hard to predict. An individual PUF circuit may be easy to make but hard to duplicate, even given the exact manufacturing process that produced it. PUF circuits may be configured to provide for example strong PUF's or weak PUF's.
A weak PUF is a circuit which produces a value, for example a key, wherein variations that occur in the production process are utilized to produce the value. The value is used in an algorithm which produces an output signal in response to an input signal, wherein the value influences the output signal.
Rather than embodying a single cryptographic key, strong PUFs implement challenge-response authentication. When a physical stimulus or input signal which corresponds to the challenge is applied to the structure, it reacts in an unpredictable way due to the complex interaction of the stimulus with the physical microstructure of the PUF circuit, wherein the reaction corresponds to the response. This exact microstructure depends on physical factors introduced during manufacture which are unpredictable. So, the strong PUF corresponds to a function which depends on the physical characteristics of the circuit providing the strong PUF.
Unclonability for example means in this context that each PUF device produces a unique and unpredictable key or has a unique and unpredictable way of mapping challenges to responses, even if it was manufactured with the same process as a similar chip or die, and it is infeasible to construct a PUF with the same key or challenge-response behavior as another given PUF because exact control over the manufacturing process is infeasible. This is because the key or the response is created by a complex interaction of the challenge with many or all of the random components. In other words, given the design of the PUF system, without knowing all of the physical properties of the random components, the keys and the responses are highly unpredictable.